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Question
Three lights flash at intervals of 5, 8 and 10 seconds respectively.
If they are all switched on at the same time, how soon after will they flash together again?
Three lights flash at intervals of 5, 8 and 10 seconds respectively.
If they are all switched on at the same time, how soon after will they flash together again?
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Solution
The correct option is B 40 seconds
To find when the three lights will flash together again, we need to find the least common multiple (LCM) of the numbers 5, 8 and 10.
Multiples of 5 = 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, ...
Multiples of 8 = 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, ...
Multiples of 10 = 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, ...
40 is the smallest multiple common to 5, 8 and 10.
∴ The three lights will flash together again after 40 seconds.
To find when the three lights will flash together again, we need to find the least common multiple (LCM) of the numbers 5, 8 and 10.
Multiples of 5 = 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, ...
Multiples of 8 = 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, ...
Multiples of 10 = 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, ...
40 is the smallest multiple common to 5, 8 and 10.
∴ The three lights will flash together again after 40 seconds.
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